To reduce the quantity of atmospheric pollutants in exhaust gases from internal combustion engines and to meet the increasingly stringent regulations regarding the content of such gases, various methods and associated apparatus have been developed to purify exhaust gases. One such method is to recycle a portion of the exhaust gas from an engine back through the engine. Particular apparatus used to practice the recycling method of emission control is described and discussed in paper No. 720123 published by the Society of Automotive Engineers, Inc. (SAE). The paper is entitled "Exhaust Recirculation and Spark Control -- A Speed Governed and Vacuum Modulated System" and is authored by E. J. Martin and D. R. Vance.
Each version of the apparatus described in SAE paper No. 720123 includes a conduit extending from an engine exhaust line to a point in the engine air/fuel intake system. A valve in the conduit regulates the flow of exhaust gas from the exhaust line to the air/fuel intake system. The valve is operated in response to an operating parameter of the engine, such as the pressure in the engine intake manifold, by an actuator device that converts the operating parameter into mechanical regulating movement. In at least one embodiment of the apparatus, the regulating valve is also initially held shut, by the operation of a speed switch and a solenoid valve, until a minimum speed is attained by the vehicle driven by the engine. The regulating valves used are either flap type or poppet type valves. As is pointed out in the paper, flap or butterfly type valves, in particular, are difficult to construct and seal.